Figuring out what's hidden in outer space between the visible stars and galaxies isn't easy.

By Emily Sohn, 00:00 AM February 9, 2004

It isn't easy to study darkness.

Try it. Next time you're outside on a clear night, look up. You might see the winking lights of an airplane, the glow of an orbiting satellite, or even the bright trail of a meteor. Of course, you'll see lots of stars.

What about all the space between the stars? Is something hidden out there in the darkness? Or is it merely empty?